In general, a hydraulic construction machine controls the flow rate of a variable displacement hydraulic pump in accordance with the operation rate of an operation lever (which means pilot signal pressure that is supplied to a spool in proportion to the operation rate of the operation lever to shift the spool that controls the flow of hydraulic fluid) in order to save energy. Further, a fixed displacement hydraulic motor is mostly used, and the flow rate that can flow into the hydraulic motor is limited to a value that is obtained by multiplying the number of revolutions by a volume of the hydraulic motor.
In such a hydraulic system, if an operator abruptly operates the operation lever to make an upper swing structure swing as shown in a graph illustrated in FIG. 1, the number of revolutions of the hydraulic motor is not sufficient and is unable to cope with the initially generated discharge flow rate of the hydraulic pump (the flow rate that actually flows into the hydraulic motor is indicated by a dotted line in the drawing).
At this time, as the pressure that flow into the hydraulic motor in the swing acceleration period is increased, a great amount of hydraulic fluid passes through a port relief valve or a main relief valve. In this case, since the generated energy is not converted into work in all, but the flow rate partially returns to the hydraulic tank through the relief valve, a loss of the flow rate (a loss of the flow rate as much as the shaded portion) occurs.